Cryptography Reference
In-Depth Information
an additional layer of complexity to the process.
Many of the basic algorithms in this topic use generic keys to con-
trol the random choicesmade by them. If an arbitrary decision needs
to be made, then a cryptographically secure random number gener-
ator driven by a key is one of the simplest mechanisms for adding a
key to the scheme.
The algorithms in Chapter 9 hide information in the least signifi-
cant bits of image and sound files by selecting a subset of elements.
This selection process is driven by a random number generator that
repeatedly hashes a key. In Chapter 13, the functions used to com-
pute the sorted list of data elements can include a key. If the same
stream of random numbers isn't available, the bits can't be extracted.
More sophisticated systems integrate the key even deeper into
the algorithm. Some try to constrain how the answer to some hard
problem is constructed. Others try to limit how it is encoded in the
data.
Many of these newer advanced systems show how just about any
computational processes can be tweaked or distorted to include a
few extra bits. Most algorithms include some arbitrary decisions
about location, order, or process and which can be driven by some
key. In the best cases, the authors understand the problem well
enough to provide some actual arguments for believing that the pro-
cess is hard to decrypt without the key.
12.3 Signing Algorithms
Many of the keying algorithms provide some kind of assurance about
the documents authenticity by acting like digital signatures for the
document. These solutions are quite useful in all situations where
digital signatures on arbitrary files provide some certainty. They're
also especially useful for watermarking. The ideal algorithm allows
the file's creator to embed a watermark in such a way that only the
proper key holders can produce that watermark.
The basic solution involves separating the image or audio file into
two parts. The first holds the details that will remain unchanged dur-
ing the steganography. If the information is hidden in the least sig-
nificant bits, then this part is the other bits, themost significant ones.
The second part is the bits that can be changed to hide information.
This set may be defined and hence controlled by a key.
A digital signature on the file can be constructed by hashing the
unchangeable part, signing the hash value with a traditional digital
signature function, and then encoding this information in the sec-
ond part reserved for hidden information. The digital signature may
